The present invention relates to epoxy resins.
Currently, epoxy resins are being used in composites in various industrial applications. For example, epoxy composites are used as circuit boards for the electronic industry and as structural members for aeronautical purposes. A critical problem in the use of epoxy resins for structural components is the extensive loss in thermal and mechanical properties (heat distortion point, strength, and modulus) when exposed to hot/wet conditions. A variety of approaches have been reported in attempts to overcome this shortcoming. Many have given a reduction in water absorption, and thus, less loss in properties in hot/wet conditions but in so doing they have compromised some of the other desirable properties such as thermal and/or oxidative stability, heat distortion temperature, toughness, and ease of formulations.
Epoxy resins made from aliphatic amines are disclosed in U.S. Pat. No. 3,843,565. Epoxy resins made from sulfonamides are disclosed in U.S. Pat. No. 3,945,973. Epoxy resins made from aromatic amines are disclosed in U.S. Pat. Nos. 2,951,822; 3,014,895; 3,312,664; 4,107,128; 4,161,588; 4,269,759; 4,487,948; 4,518,786; 4,540,769; and 4,560,739. U.S. Pat. No. 4,540,769 teaches epoxy resins made from aromatic amines such as bis(4-aminophenyl)ether.
One commercial epoxy resin is tetraglycidyl-4,4'-(4-aminophenyl)-p-diisopropylbenzene which has the following structure as disclosed in U.S. Pat. No. 4,680,341: ##STR2## This resin is commercially available from the Shell Chemical Company as EPON HPT 1071.
Another commercial epoxy resin is tetraglycidyl-4,4'-(3,5-dimethyl-4-aminophenyl)-p-diisopropylbenzene which has the following structure: ##STR3## This resin is commercially available from the Shell Chemical Company as EPON HPT 1072.
A widely used epoxy resin in the aeronautical industry is tetraglycidyl 4,4'-diamino diphenyl methane which has the formula: ##STR4## This epoxy resin is commercially available from Ciba-Geigy Corporation as ARALDITE (registered trademark) MY-720. As the resin is prepared by reacting epichlorohydrin with methylene dianiline, it is frequently identified as tetraglycidylated methylene dianiline (hereinafter TGMDA).
While each epoxy esin discussed above has a number of desirable properties, each is deficient in one or more properties desired in the art. For example, the cured TGMDA resins are extremely moisture sensitive, and hence, subject to degradation of properties at high humidities. The impact strength and toughness of these resins are lower than those obtainable with other resins systems.
Many of the cured epoxy resins discussed above absorb substantial quantities of moisture from the atmosphere. A high moisture content significantly lowers the glass transition temperature (Tg) of the cured resins as compared with the values obtained with the bone dry resins. As a result, the decrease of the glass transition temperature decreases the effective maximum temperature at which the cured epoxy resins and composites prepared therefrom can be employed. The shortcomings of the state-of-the-art epoxy resins are discussed in the following references: Apicella et al, I and E Chem.: Prod. Dev., 288 (1984); Browning, Polymer Eng. Sci., 18, 16 (1978); McKague, J. Appl. Polymer Sci., 22, 1643 (1978); Ellis et al, Polymer 25(2), 664 (1984); and Bauer, 31st International SAMPE Symposium (1986).